1. Technical Field
This invention relates generally to a system for decontaminating a contaminated volume of soil, and more particularly to an enhanced pressure gradient remediation system.
2. Discussion
Soil and groundwater purity has become an important environmental issue. Enormous amounts of contaminants now pollute numerous toxic sites. Spurred by federal legislation, a massive clean-up is underway.
A contaminated volume of soil is generally characterized by trapped lenses of contaminants located above the water table level, at or near the water table level, or below the water table level. There may also be residual contamination of lower concentrations of contaminants trapped within the pores of the soil. Liquid contaminants which are heavier than water move downward into the capillary fringe in the water table while liquids lighter than water tend to spread laterally when they encounter the fringe of the water table.
One method for cleaning up contaminated sites consists of removal of the contaminated soil. This method is bulky, impractical and slow. Another method is groundwater extraction to remove residual contamination. Groundwater withdrawal is intended to depress the water table in order to recover the spill, such as gasoline, in a separate phase. However, groundwater extraction is inefficient. However, the amount of free product typically recovered by this method is significantly less than the estimated amount of the spill. It cannot be predicted beforehand how much water must be pumped, for how long, or whether the pumping will be effective. Groundwater extraction may also require additional equipment such as down-hole pumps or entrainment tubes.
Contaminated soil is typically decontaminated by installing at least one permanent extraction well connected to a vacuum pressure. This vacuum pressure is typically measured in terms of inches of water. This type of vacuum extraction is effective on volatile compounds, such as gasoline or solvents, and partially effective on semi-volatile compounds, such as diesel fuel. The effectiveness of a vacuum extraction system on semi-volatile compounds is on the order of 30%. Pure vacuum extraction only volatilizes compounds with particular vapor pressures, or Henry's constants. Typical in-situ vacuum extraction systems have limited success in finely-grained soils, require over 12 months of operating time and typically can only reduce concentrations of volatile contaminants in the soil to approximately 10 parts per million.
Another technique is the injection of steam into the sub-surface to create a positive pressure gradient as well as increased sub-surface soil temperature to remove volatile and semi-volatile compounds. Steam enhanced extraction can remove other, less volatile compounds than vacuum extraction alone, but has not utilized high vacuum to forcibly assist the migration of sub-surface contaminants toward the extraction wells. The steam injection is also generally performed through permanent wells.
As a result, existing technologies have required enormous capital expenditures for permanent wells, treatment of the entire contaminated area at the same time, and over 12 months of operating time. Permanent wells are more costly to install, more difficult and costly to abandon, and limit flexibility.